The present disclosure relates to an image processing apparatus and an image processing method for compressing image data by combining spatial frequency conversion, quantization, and entropy coding.
Compression technologies for the image data based on a combination of the spatial frequency conversion, the quantization, and the entropy coding, such as a Joint Photographic Experts Group (JPEG) format and a JPEG 2000 format are widespread.
In general, in the JPEG format, an ability to compress data or a “compressibility” can be changed as necessary by changing a quantization table. In a normal state, when the compressibility is increased, image quality may deteriorate after expanding the image, otherwise referred to as an “expansion.”
In one technology, a quantization table may be changed so as to increase the compressibility. In particular, the change may reduce an encoded data size while suppressing deterioration of the image quality.
Alternatively, in another technology, a quantization factor is changed by changing a scaling factor for each pixel block based on image contents, to thereby increase the compressibility.
In some instances, the quantization factor may be changed for each pixel block. Hence, it maybe difficult to reduce the encoded data size in consideration of a change in the image contents. In particular, the change may be within a range that is spatially shorter than a size of the pixel block. Further, the quantization may correspond to the change in the image contents within a range. In some instances, this range may be spatially shorter than a size of the pixel block and the quantization may not be performed, which is not preferred for image quality.